Field of the Invention
The present invention relates to an image processing apparatus which performs halftone processing.
Description of the Related Art
An image processing apparatus receives print data from a host computer (PC) or the like, and generates a raster image capable of print processing from the print data. Then, the image processing apparatus performs image processing (to be referred to as “output correction” hereinafter) on RGB data (for example, 8 bits for each color: 256 tones), which is the raster image, to match a characteristic (to be referred to as an “output characteristic” hereinafter) unique to the output unit of the image processing apparatus. The image processing apparatus converts the number of tonality of the RGB data into the number of tonality reproducible by the output unit (to be referred to as “halftone processing” hereinafter), and prints. That is, multiple tones are expressed by color materials of several colors using an area coverage modulation method of changing the number of dots to be formed per unit area by halftone processing.
To synchronize with the operation of the output unit during the above-described processing, the image processing apparatus temporarily stores intermediate data in a storage (for example, a hard disk or dynamic random access memory (DRAM)) in the image processing apparatus. The intermediate data storage method includes a multi-value storage method of storing multi-value RGB data before halftone processing, and a binary storage method of storing binary data after halftone processing.
The binary storage method decreases the amount of data to be stored in comparison with the multi-value storage method, and can reduce the cost by reducing the storage capacity of the storage. However, halftone processing expresses tones by the area coverage modulation method. Thus, the information amount of binary data becomes smaller than that of multi-value data, decreasing the resolution of a character or line art. Note that “decreasing the resolution” means that an image blurs or crushes (for example, degradation of the edge of a character or line art).
The output characteristic depends on the remaining amount of color material and the type of printing medium, and represents a temporal change upon variations of the temperature and humidity of the output unit. Output correction comprises gamma correction processing of correcting density variations arising from, for example, a device difference. The gamma correction processing is processing on multi-value data, and it is difficult to perform the gamma correction processing on binary data after halftone processing. For this reason, when the output characteristic changes upon storing intermediate data, it is difficult for the binary storage method to obtain a high-quality printed image.
There is proposed a technique in which multi-value data and binary data are combined, the amount of data to be stored is decreased in comparison with the multi-value storage method, and output correction is possible on stored data. This technique performs image separation to separate print data into a character or line art (to be referred to as a “character/line art region” hereinafter) and an image having tonality (to be referred to as an “image region” hereinafter). The character/line art region is stored as high-resolution binary data without performing halftone processing. As for the image region, only part of multi-value data is stored as multi-value data in association with halftone dot position information and the size.
That is, the above-described technique tries to obtain a high-quality output result by performing processing complying with the image characteristic while reducing the amount of data to be stored. To obtain a high-quality printout by this technique, the image separation result is important. It is however difficult to always obtain satisfactory image separation results for various images. Especially, it is difficult to accurately separate an image region from an image obtained by reading a halftone image original by a scanner or the like.
In the above-described technique, only information (to be referred to as “halftone dot information” hereinafter) expressing each halftone dot is held for a region determined to be an image region by image separation. A halftone image is generated based on the held halftone dot information. Since halftone dot information is held for each halftone dot, the data amount is reduced, but the resolution still decreases.